Insulating material



INSULATING MATERIAL No Drawing. Application October 11, 1954 Serial No.461,650

23 Claims. (Cl. 154-25) This invention relates to phenolic resininsulation material having extremely high insulation resistance which isretained under high humidity conditions. While the utility of thematerial is particularly derived from such high insulation resistanceproperties, it is capable of other uses where high insulation resistancemay be relatively unimportant. While the material is produced throughthe use of a laminating procedure, the final material is homogeneous toa high degree.

Phenolic resin-paper laminates are in wide use in electrical apparatusto serve as supports or merely for the insulation of conductive elementsof electrical apparatus. Provided generally in sheet or block form, suchmaterial is punched, or otherwise machined, to provide the necessarymechanical configurations for supporting and/ or insulating use.Metallic elements of electrical circuits are secured thereto byriveting, clamping or other means and, in recent years, sheets of suchmaterial have been used as bases for printed circuits. In most suchuses, high insulation resistance is desired, and particularly themaintenance of high insulation resistance under high humidityconditions. The merit of such materials has generally been tested byproviding electrodes in three-sixteenth inch holes in the materialhaving one inch center to. center spacing in a two inch by three inchsheet regardless of thickness. The majority of such materials used inthe past show, under such test, a very high resistance of the order of500,000 megohms under dry conditions but with considerable loss ofresistance with humidity, the resistance dropping, for example, to 1000megohms. Prior to the present invention, the best materials from aninsulation standpoint attained a resistance under such test of about70,000 megohms under 90% humidity conditions. In accordance with thepresent invention, there may be attained under 90% humidity conditionsresistances in such test ranging from 200,000 to 1,000,000 megohms. Thematerial provided in accordance with the invention shows very littledeterioration with moisture and quick recovery on drying in a dryatmosphere. It has been commonly assumed in the past that highinsulation resistance is dependent upon very low water absorption. Thepresent material has low absorption as compared with the majority ofother insulation materials of equivalent uses presently on the market,but the water absorption is not, in contrast with such materials,extremely low. Actually, the improved material shows a much higherresistance under high humidity conditions than materials havingsubstantially lower water absorption. It may, therefore, be consideredopen to question whether water absorption is a prime factor indetermining maintenance of high resistance under humidity conditions. 1

The objects of the invention relate to attainment of high insulationmaterial, as already indicated, and particularly material which issusceptible to punching for the purpose of formulation of insulatingsupports or other elements in electrical apparatus, including theformation of insulating bases for printedcircuits. These gennited StatesPatent eral objects as well as others which will appear hereafter, andthe attainment of the results of the invention, may be best made clearby description of several specific formulation procedures followed by adiscussion of variations which may be introduced into the formulationprocess.

In a typical process there is introduced into a kettle a batch ofingredients as follows:

3200 parts by weight of a commercial cresol consisting of a blend ofortho, meta and para cresols such as is commonly used in the art for theproduction of thermosetting phenolic resins, e.g. containing 17%o-cresol, 44% m-cresol, 25% p-cresol and 14% low boiling xylenols.

400 parts by weight of a phenol having substituted in its meta positiona long chain, such phenol being derived from cashew nut shell oil andhaving the formula C H (OH) 'C H (This phenol is fully hydrogenated inits side chain but incompletely hydrogenated phenols may be used.)

720 parts by weight of paraformaldehyde. 1300 parts of 37% aqueousformaldehyde. 27 parts by weight of hydrated lime.

These ingredients are charged into the kettle in the order listed andthe batch is held in the temperature range of 60 F. to 70 F. until aviscosity of Gardner B to E, preferably the latter, is reached. Thistakes a period of about five to six days. There are then added 210 partsby weight of 42% aqueous ammonium sulfide. After this addition, thebatch is allowed to age for twenty-four hours and there are then added1060 parts by weight of tricresyl phosphate and 262 parts by weight ofchlorinated diphenyl havinga boiling range from 290 C. to 420 C. (Bychlorinated diphenyl are meant mixtures resulting from the chlorinationof diphenyl, such as the Aroclors marketed by Monsanto ChemicalCompany.) The last two ingredients are not soluble in the mixturetheretofore in the kettle so that the batch must be agitatedcontinuously to keep them suspended.

The resulting composition is then coated on an absorbent rag paper underconditions which may be typically the following:

The paper is advanced at the rate of 20 to 30 feet per minute. Heat isapplied so that at the entrance end of the coater the temperature isapproximately 250 F. and at the dry end the temperature is about 325 F.,the coating being applied to the paper while moving to get a minimum ofsoftening, while flashing on the water in the coater, to maintainstrength. This procedure typically gives a resin content of 62% byweight, this being the amount of resin which may be exuded underapplication of heat and pressure.

The resulting impregnated paper sheets may be superimposed to secure thedesired ultimate thickness of the product and are pressed at 1800 poundsper square inch at a temperature of 275 to 350 F. for a period rangingfrom 50 to 60 minutes. The finished product, if about in thickness, isthen subjected to a temperature of 275 to 300 F. for a period of about 1/2 hours to obtain the optimum electrical properties. If the finishedproduct is thicker the time of this last treatment must be increased.

As a variation of the foregoing there may be charged into the kettle4500 parts by weight of the cresol mixture, 1017 parts by weight ofparaformaldehyde, 1278 parts by weight of aqueous 37% formaldehyde, and31 parts by weight of hydrated lime. With this composition the batch isheld in the range of 60 F. to F. until a viscosity of Gardner A to D(preferably about Gardner C) is reached. There are then added 240 partsby weight of 42% aqueous ammonium sulfide and the batch is held foratleast 24 hours before it is used. Following this there are added to 1000parts by weight of the foregoing batch 168 parts by weight of tricresylphosphate and 42 parts by weight of chlorinated diphenyl of the typedescribedabove. Agitation and coating may be carried out as describedpreviously with the attainment of an exudable resin content in the ragpaper of 61 to 63%. The final pressing after lamination is carried outas described above followed by similar curing.

While reference has been made to a laminating procedure involving thesuperposition of the resin coated sheets followed by pressing atelevated temperatures, in the final product the fibrous structuredisappears, there being little evidence of lamination in the finalproduct which is translucent. The electrical properties are uniformthroughout the final product, i.e. there is not merely a facing sheethaving the electrical resistance properties as in some prior laminatedresinous products.

Numerous variations of the procedures described may be adopted. Firstthere is a substantial range of varied compositions of the phenol orphenol mixture which may be used in the batch and typified above by thecresol mixture. Numerous mixtures of ortho, meta and para cresols may beused or meta cresol may be used alone or with only one of the pair ofortho and para cresols. Desirably, however, when cresols are used, metacresol is present in substantial amounts. Various xylenols andresorcinols as well as other phenols may be used in admixture withcresols to give highly satisfactory results. However, with phenol alonethe results are not impressive, and it appears that the highersubstituted phenols are required to secure results which may beconsidered outstanding from the standpoint of securing products havinghigh insulation resistance.

It will be noted that in the above described formula tions bothcommercial 37% aqueous formaldehyde and paraformaldehyde are added. Forbest results, a formaldehyde concentration higher than the commercial37% aqueous solution is desirable and this concentration is increased bythe use of the paraformaldehyde which represents 90 to 92% availableformaldehyde. An appreciable amount of Water is desirably present, andfor this reason it is desirable to have present some substantial amountof the aqueous formaldehyde as well as paraformaldehyde.

In the first formulation mentioned above, reference has been made to thepresence in the mixture of a long chain meta substituted phenol. This isa desirable but not a necessary constituent of the batch and otherdesirable added .phenols comprise such substituted ones as para nonylphenol, paratertiary 'butyl phenol, or the like. The long chainsubstituted phenol is added for internal plasticizing so that theproduct may be punched. Such phenols do not modify substantially theelectrical properties.

The hydrated lime, for which may be substituted barium or strontiumhydroxide, apparently acts primarily as a catalyst and may be acommercial hydrated lime but, preferably, one which is high in calciumcontent, for example, containing around 90% calcium hydroxide and 10% orless of magnesium oxide. The amount of hydrated lime maintains the pH ofthe mixture in the range of about 8.5 to 9.8. If the pH is below 8.5,the reaction takes place but becomes impractically slow. The temperatureis desirably held in the range from about 60 F. to 80 F. Desired in thisreaction is the formation of phenol alcohols without resinification. Ifthe temperature rises too high, resinification takes place whereas thisis desired to a substantial degree only following the addition ofsulfide. The control by determination of viscosity gives a goodindication of the phenol alcohol formation and, in general, viscositiesof Gardner A to E indicate the completion of the preliminary reaction,the particular viscosity used as an indication being dependent upon theparticular materials involved. As

used in this specification and claims the term phenol alcohols is usedto designate the products formed by the reaction of phenols andformaldehyde under conditions typified 'by those described above.

It may be noted that in making generally similar products in the pastthe reaction has not been controlled in the fashion outlined to end withthe formation of phenol alcohols, but rather the reaction has beencarried out to secure substantially complete resinification. I

An essential part of the procedure is the involvement of the addition ofsulfide for completing the reaction. The particular sulfide added is notimportant from the reaction standpoint but to maintain the highestinsulation resistance it is desirable not to incorporate metallic ions.When maximum insulation resistance is desired, therefore, it isadvantageous to avoid the use of metal sulfides such as those of sodiumor potassium. Using ammonium sulfides, the ammonia disappears in thecoating or other subsequent heating leaving possibly conductive ionsabsent. Hydrogen sulfide is the essential addition and since addition ofthis in gaseous form would necessitate closed containers anddifficulties of introduction, the practical way to add sulfide isthrough an ammonium sulfide. Desirably, this is done by adding aqueousammonium sulfide which represents a high percentage of availablehydrogen sulfide. However, it is possible to use yellow sulfides, i.e.various polysulfides. Furthermore, hydrogen sulfide may be introduced,though as stated, this involves some procedural disadvantages.

The reactions which occur when the ammonium or other sulfide is addedare difficult to determine. However, the products formed from thatreaction, in conjunction with the water present, have a very definiteaflinity for cellulose and seem to attack and swell cellulose and changeit markedly. The same reactions do not take place when ammonium sulfideis added to a cresolformaldehyde resin made by the normal procedure ofrefluxing, even when the water, which was present in the formaldehyde,is not removed.

Tricresyl phosphate acts not only as a plasticizer but also appears tolower the water absorption characteristics of the final product thoughexcessive amounts are to be avoided. In place of tricresyl phosphatethere may be used triphenyl phosphate, cresyl diphenyl phosphate,chlorinated triphenyl phosphate, chlorinated tricresyl phosphate,chlorinated diphenyl of the type mentioned above, or xylenyl phosphates.Phthalate plasticizers used alone are ineffective but they may be addedin order to improve the punching characteristics of the final product.The tricresyl phosphate or the equivalent is not merely used for itsplasticizing action but gives flame retarding results as well as proofagainst fungus growth and high temperature stability as well as aidingin maintenance of the emulsion which is applied to the paper. Melamineresins may be used in conjunction with tricresyl phosphate or anotherplasticizer to impart properties of higher are resistance and flameretardancy without affecting other electrical properties. The additionof chlorinated diphenyl as described above in the specific examples isadvantageous but not essential, though, as above indicated, chlorinateddiphenyl may be the sole plasticizer added. The amounts of plasticizersused are subject to wide variations, depending entirely upon theproperties desired in the final product.

The paper which is used is desirably of thicknesses ranging fromtwo-thousandths of an inch to twentythousandths of an inch. The water isremoved from the paper in the drying oven forming part of the coater. Inthe coating process water is removed and the alcohols are condensed toform resins. Various papers may be used, including kraft as well ascotton rag or linter paper.

The sheets of material as taken from the press are desirably post curedin an oven as described above to remove sulphur-containing gases and toget the maximum resistance. The products provided .in accordance withthe invention have a low dielectric constant, lowpower factor, and highdielectric str.:ngth. It may be here noted that dielectric constant anddielectric strength are distinct properties, the former referring to theconstant which appears in determining condenser capacity, and dielectricstrength referring to resistance against breakdown under highpotentials. The post curing is highly desirable to secure the ultimatebest qualities in all of these respects.

The temperatures involved in the coater and in the post curing and thetimes there involved are not critical. Desirably, however, thetemperature in the coater is maintained rather high as indicated in theexamples above to secure flash removal of water so as to maintain thewet strength of the sheet to avoid breakage, the water being so rapidlyremoved that within a short period of passage the sheet becomessufficiently strong by reason of the resinification.

In the curing procedure, the desirable result is the elimination ofsulphur gases including ammonium sulfide and the ammonia contentintroduced with the sulfide. The temperature involved in this postcuring is subject to wide variation, lower temperatures requiring longertimes and vice versa, the upper limit of temperature being determined byavoidance of heat damage by carbonization.

What is claimed is:

l. A method of preparing insulating material comprising maintaining anadmixture of at least one cresol, a phenol having an aliphatic sidechain of at least four carbon atoms, formaldehyde and an alkali earthhydroxide over an extended period at a temperature sufiiciently low toprevent substantial resinification until the mixture attains a viscosityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said mixture a material providing hydrogen sulfide, and addingtricresyl phosphate and chlorinated diphenyl, then coating papertherewith within a temperature range in excess of the boiling point ofwater, and laminating sheets thus coated under high pressure and atelevated temperature.

2. A method of preparing insulating material comprising maintaining anadmixture of at least m-cresol, a phenol having an aliphatic side chainof at least four carbon atoms, formaldehyde and an alkali earthhydroxide over an extended period at a temperature sufficiently low toprevent substantial resinification until the mixture attains a viscosityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said mixture a material providing hydrogen sulfide, and addingtricresyl phosphate and chlorinated diphenyl, then coating papertherewith within a temperature range in excess of the boiling point ofwater, and laminating sheets thus coated under high pressure and atelevated temperature.

3. A method of preparing insulating material comprising maintaining anadmixture of at least one cresol, a phenol having an aliphatic sidechain of at least four carbon atoms, formaldehyde and an alkali earthhydroxide over an extended period at a temperature suffieiently low toprevent substantial resinification until the mixture attains a viscosityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said mixture a material providing hydrogen sulfide, thencoating paper therewith within a temperature range in excess of theboiling point of water, and laminating sheets thus coated under highpressure and at elevated temperature.

4. A method of preparing insulating material comprising maintaining anadmixture of at least m-cresol, a phenol having an aliphatic side chainof at least four carbon atoms, formaldehyde and an alkali earthhydroxide over an extended period at a temperature sufiiciently low toprevent substantial resinification until the mixture attains a viscosityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said mixture a material providing hydrogen sulfide, thencoating paper therewith within a temperature range in excess of theboiling point of water, and laminating sheets thus coated under highpressure and at elevated temperature.

5. A method of preparing insulating material comprising maintaining anadmixture of at least one cresol, a phenol having an aliphatic sidechain of at least four carbon atoms, formaldehyde and an alkali earthhydroxide over an extended period at a temperature sufliciently low toprevent substantial resinification until the mixture attains a viscosityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said mixture a material providing hydrogen sulfide, and addinga plasticizer, then coating paper therewith Within a temperature rangein excess of the boiling point of water, and laminating sheets thuscoated under high pressure and at elevated temperature.

6. A method of preparing insulating material comprising maintaining anadmixture of at least m-cresol, a phenol having an aliphatic side chainof at least four carbon atoms, formaldehyde and an alkali earthbydroxide over an extended period at a temperature sufficiently low toprevent substantial resinification until the mixture attains a viscosityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said 7 mixture a material providing hydrogen sulfide, andadding a plasticizer, then coating paper therewith within a temperaturerange in excess of the boiling point of water, and laminating sheetsthus coated under high pressure and at elevated temperature.

7. A method of preparing insulating material comprising maintaining anadmixture. of at least one cresol, formaldehyde and an alkali earthhydroxide over an extended period at a temperature sufficiently low toprevent substantial resinification until the mixture attains a viscosityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said mixture a material providing hydrogen sulfide, thencoating paper therewith within a temperature range in excess of theboiling point of water, and laminating sheets. thus coated under highpressure and at elevated temperature.

8. A method of preparing insulating material comprising maintaining anadmixture of at least one cresol, formaldehyde and an alkali earthhydroxide over an extended period at a temperature sufiiciently low toprevent substantial resinification until the mixture attains a viscosityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said mixture a material providing hydrogen sulfide, adding aplasticizer to said mixture, then coating paper therewith Within atemperature range in excess of the boiling point of water, and

, laminating sheets thus coated under high pressure and at elevatedtemperature.

9.A method of preparing insulating material comprising, maintaining anadmixture of at least m-cresol, formaldehyde and an alkali earthhydroxide over an extended period at a temperature sufiiciently low toprevent substantial resinification until the mixture attains a viscosityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said mixture a material providing hydrogen sulfide, thencoating paper therewith within a temperature range in excess of theboiling point of Water, and laminating sheets thus coated under highpressure and at elevated temperature.

10. A method of preparing insulating material comprising maintaining anadmixture of at least m-cresol, formaldehyde and an alkali earthhydroxide over an extended period at a temperature sufliciently low toprevent substantial resinification until the mixture attains a viscosityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said mixture a material providing hydrogen sulfide, adding aplasticizer to said mixture, then coating paper therewith within atemperature range in excess of the boiling point of water, and

7 laminating sheets thus coated under high pressure and at elevatedtemperature.

11. A method of preparing insulating material comprising maintaining anadmixture of at least one cresol, a phenol having an aliphatic sidechain of at least four carbon atoms, aqueous formaldehyde andparaformaldehyde and an alkali earth hydroxide over an extended periodat a temperature sufficiently low to prevent substantial resinificationuntil the mixture attains a viscosity in the range of Gardner A to E,thereby to form phenol alcohols, then adding to said mixture a materialproviding hydrogen sulfide, then coating paper therewith within atemperature range in excess of the boiling point of water, andlaminating sheets thus coated under high pressure and at elevatedtemperature.

12. A method of preparing insulating material comprising maintaining anadmixture of at least one cresol, formaldehyde and an alkali earthhydroxide over an extended period at a temperature sufiiciently low toprevent substantial resinification until the mixture attains a viscosityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said mixture an aqueous solution of ammonium sulfide, thencoating paper therewith within a temperature range in excess of theboiling point of water, and laminating sheets thus coated under highpressure and at elevated temperature.

13. A method of preparing insulating material comprising maintaining anadmixture of at least one cresol, formaldehyde and an alkali earthhydroxide over an extended period at a temperature sufiiciently low toprevent substantial resinification until the mixture attains a viscisityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said mixture an aqueous solution of ammonium sulfide, adding aplasticizer to said mixture, then coating paper therewith within atemperature range in excess of the boiling point of water, andlaminating sheets thus coated under high pressure and at elevatedtemperature.

14. A method of preparing insulating material comprising maintaining anadmixture of at least m-cresol, formaldehyde and an alkali earthhydroxide over an extended period at a temperature sufiiciently low toprevent substantial resinification until the mixture attains a viscosityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said mixture an aqueous solution of ammonium sulfide, thencoating paper therewith within a temperature range in excess of theboiling point of water, and laminating sheets thus coated under highpressure and at elevated temperature.

15. A method of preparing insulating material comprising maintaining anadmixture of at least m-cresol, formaldehyde and an alkali earthhydroxide over an extended period at a temperature sufliciently low toprevent substantial resinification until the mixture attains a viscosityin the range of Gardner A to E, thereby to form phenol alcohols, thenadding to said mixture an aqueous solution of ammonium sulfide, adding aplasticizer to said mixture, then coating paper therewith within atemperature range in excess of the boiling point of water, and

laminating sheets thus coated under high pressure and at elevatedtemperature.

16. An insulating material comprising the product of the lamination withthe application of heat and pressure of sheets of paper impregnated witha resin produced by the reaction of at least one phenol alcohol with asulfide capable of providing hydrogen sulfide.

17. An insulating material comprising the product of the lamination withthe application of heat and pressure of sheets of paper impregnated witha resin produced by the reaction of at least one phenol alcohol with asulfide capable of providing hydrogen sulfide to which a plasticizer hasbeen added.

18. An insulating material comprising the product of the lamination withthe application of heat and pressure of sheets of paper impregnated witha resin produced by the interaction of at least one cresol, formaldehydeand an alkali earth hydroxide followed by reaction of the resultingmixture with a sulfide capable of providing hydrogen sulfide.

19. An insulating material comprising the product of the lamination withthe application of heat and pressure of sheets of paper impregnated witha resin produced by the interaction of at least one cresol, formaldehydeand an alkali earth hydroxide followed by reaction of the resultingmixture with a sulfide capable of providing hydrogen sulfide to which aplasticizer has been added.

20. An insulating material comprising the product of the lamination withthe application of heat and pressure of sheets of paper impregnated witha resin produced by the reaction of at least one phenol alcohol withammonium sulfide.

21. An insulating material comprising the product of the lamination withthe application of heat and pressure of sheets of paper impregnated witha resin produced by the interaction of at least one cresol, formaldehydeand an alkali earth hydroxide followed by reaction of the resultingmixture with ammonium sulfide.

22. An insulating material comprising the product of the lamination withthe application of heat and pressure of sheets of paper impregnated witha resin produced by the interaction of at least one cresol, a phenolhaving an aliphatic side chain of at least four carbon atoms,formaldehyde and an alkali earth hydroxide followed by reaction of theresulting mixture with a sulfide capable of providing hydrogen sulfide.

23. An insulating material comprising the product of the lamination withthe application of heat and pressure of sheets of paper impregnated witha resin produced by the interaction of at least one cresol, a phenolhaving an aliphatic side chain of at least four carbon atoms,formaldehyde and an alkali earth hydroxide followed by reaction of theresulting mixture with ammonium sulfide.

' References Cited in the file of this patent UNITED STATES PATENTS

1. A METHOD OF PREPARING INSULATING MATERIAL COMPRISING MAINTAINING ANADMIXTURE OF AT LAST ONE CRESOL, A PHENOL HAVING AN ALIPHATIC SIDE CHAINOF A LAEAST FOUR CARBON ATOMS, FORMALDEHYDE AND AN ALKALI EARTHHYDROXIDE OVER AN EXTENDED PERIOD AT A TEMPERATURE SUFFICIENTLY LOW TOPREVENT SUBSTANTIAL RESINIFICATION UNTIL THE MIXTURE ATTAINS A VISCOSITYIN THE RANGE OF GARDNER A TO E, THEREBY TO FORM PHENOL ALCOHOLS, THENADDING TO SAID MIXTURE A MATERIAL PROVIDING HYDROGEN SULFIDE, AND ADDINGTRICRESYL PHOSPHATE AND CHLORINATED DIPHENYL, THEN COATING PAPERTHEREWITH WITHIN A TEMPERATURE RANGE IN EXCESS OF THE BOILING POINT OFWATER, AND LAMINATING SHEETS THUS COATED UNDER HIGH PRESSURE AND AELEVATED TEMPERATURE